Plug closure in a container for subjecting sample to shock wave

ABSTRACT

A PLUG CLOSURE FOR CLOSING THE END OF A CYLINDRICAL CONTAINER, E.G., ONE END OF A CIRCULAR METAL CYLINDER, CONTAINING A SAMPLE THAT IS BEING SUBJECTED TO A SHOCK WAVE, WHICH RETAINS THE SAMPLE WHILE PERMITTING THE SHOCK WAVE TO LEAVE IT BY PROPAGATING INTO THE CLOSURE ACROSS THE SAMPLE/CLOSURE INTERFACE, COMPRISING A FIRST SECTION IN CONTACT WITH THE SAMPLE, WHICH IS OF ABOUT THE SAME SHOCK IMPEDANCE AS THE SAMPLE AT THE INTERFACE THEREWITH AND WHICH IS SOLID AFTER PASSAGE OF THE SHOCK WAVE THROUGH SAID SECTION AND OF SUFFICIENT STRENGTH TO RETAIN THE SAMPLE IN THE CONTAINER? AND A SECOND SECTION IN CONTACT WITH THE SURFACE OF THE FIRST SECTION OPPOSITE THAT WHICH CONTACTS THE SAMPLE, THE FIRST AND SECOND SECTIONS HAVING ABOUT THE SAME SHOCK IMPEDANCE AT THEIR INTERFACE, AND THE SECOND SECTION CARRYING OFF, BY SPALLING, MOMENTUM ASSOCIATED WITH THE SHOCK WAVE.

' March9, 1 G. R. COWAN 3,568,248

' v PLUG CLOSURE IN A CONTAINER FOR SUBJECTING SAMPLE T0 SHOCK WAVE Filed March '4, 1969 IIIIIIIIII/Il/ III/l INVVENTOR BY GEORGE R. com

ATTORNEY U.S. Cl. 185

United States Patent Ofifice 3,568,248 Patented Mar. 9, 1971 3,568,248 PLUG CLOSURE IN A CONTAINER FOR SUB- JECTING SAMPLE T SHOCK WAVE George R. Cowan, Woodbury, N.J., assignor to E. I. du Pont de Nemours and Company, Wilmington, Del. Filed Mar. 4, 1969, Ser. No. 804,199 Int. Cl. B30b 11/00, /00, 15/00 ABSTRACT OF THE DISCLOSURE A plug closure for closing the end of a cylindrical container, e.g., one end of a circular metal cylinder, containing a sample that is being subjected to a shock wave, which retains the sample while permitting the shock wave to leave it by propagating into the closure across the sample/closure interface, comprising a first section in contact with the sample, which is of about the same shock impedance as the sample at the interface therewith and which is solid after passage of the shock Wave through said section and of suflicient strength to retain the sample in the container; and a second section in contact with the surface of the first section opposite that which contacts the sample, the first and second sections having about the same shock impedance at their interface, and the second section carrying oflf, by spalling, momentum associated with the shock wave.

BACKGROUND OF THE INVENTION This invention relates to a novel plug closure for closing the end of a cylindrical container, e.g., a circular metal cylinder, containing a sample that is being subjected to a shock wave.

In recent years there has been considerable interest in the development of techniques for subjecting condensed media, usually solid materials, to shock pressures. Among the objectives of shock pressure treatment are the densification of porous materials, modification of the properties and microstructure of materials, and the production of phase changes and chemical reactions.

The application of shock pressure to a specimen of a practical finite side involves the motion of a pressure wave through the specimen. The application of this pressure can be accomplished, for instance, by detonation of an explosive adjacent to the condensed material or a container therefor, e.g., as described in U.S. Pat. 3,022,544, or by means of the impact of a projectile or driver with the material or a container therefor, e.g., as described in the co-pending, co-assigned application Ser. No. 804,230 filed Mar. 4, 1969, by George R. Cowan and Anthony S. Balchan, wherein a sample is subjected to shock pressure by progressively collapsing an explosively propelled circular metal cylinder onto the sample in the form of a circular cylinder, and wherein containment for the shocked material can be provided by the propelled cylinder, or a double-walled cylinder formed by an initial containing cylinder and the propelled cylinder.

In applying techniques which require containment for the shocked sample, such as the cylindrical containment referred to above, difliculty is often encountered in pro- 12 Claims enough to require strong containment to brin the sample to rest. Even a small opening in the end closure can result in a complete loss of the sample.

Further difliculty is caused by the reflection of shock waves. Collision of the Waves with boundaries of different impedance generates reflected waves which travel back into the material. Reflection of the pressure wave from a free end can cause a net tension to be generated in the interior of the sample, causing disruption of it. Impact of the shock wave against a higher-impedance body, e.g., a solid steel plug, would create a pressure which is much higher than the incident shock pressure, and could cause the container to burst open. Also, if a higher-impedance plug were gripped by the container, as is the case with the propelled cylinder in theabove-described technique, forward motion could cause the container to fail in tension. Thus, a suitable end closure must be capable of transmitting the shock wave with a minimum of reflected waves.

Accordingly, there is needed an end closure for cylindrical containers containing samples that are being subjected to a shock wave moving in the direction of the cylinders longitudinal axis which retains the sample while at the same time carrying off the momentum associated with the high-pressure shock wave with a minimum of reflected waves.

SUMMARY OF THE INVENTION In accordance with the present invention, there is provided a plug closure for closing the end of a container containing a sample that is being subjected to a shock wave which retains the sample while transmitting the shock pressure pulse with a minimum of reflected waves. The plug closure comprises (a) a first section, in contact with the sample, having about the same shock impedance as the sample at the interface therewith at the shock pressure employed, and comprising a material which, after the passage therethrough of the shock wave, will be solid and of suflicient strength to retain the sample in the container; and

(b) a second section, in contact with the first section at the surface opposite that which contacts the sample, having a shock impedance at the interface with the first section of about that of the first section at the interface with the second section at the shock pressure employed, and comprising material that will carry off, by spalling, momentum associated with the shock wave employed.

Shock impedance is equal to the initial density of a material times the velocity of a shock Wave passed through it.

The first section can have a gradually decreasing porosity from the surface in contact with the sample to the surface in contact with the second section, With the portion in contact with the sample having a bulk density and shock impedance of about that of the sample at the interface. The second section, preferably, has a gradually increasing porosity ranging from a porosity at which its shock impedance at its interface with the first section is about that of the first section at the same interface, to a lower value at the surface opposite the first section.

Since containment of a shocked material is more easily accomplished with containers having a minimum of closure surface area exposed to the shock wave, cylindrical containers are preferred. In such cases, the plug closure is cylindrical and is required at only one end of the container if the shock wave travels in one axial direction. A circular cross-section is preferred in the container cylinder, and, therefore, in the plug closure, to provide pressure uniformity.

I o BRIEF DESCRIPTION OF THE DRAWING In the accompanying drawing, FIG. 1 represents a vertical cross-sectional view of a representative shock pressure assembly employing the novel plug closure of this invention. FIGS. 2 and 3 represent sections of the assembly of FIG. 1 drawn to a larger scale, the sections being taken at lines 22 and 33, respectively.

DESCRIPTION OF THE PREFERRED EMBODIMENTS The parameters of the plug closure of this invention vary according to the particular shock pressure technique to be employed, i.e., the pressures and particle velocities to be attained. As the pressures and particle velocities are increased, the retaining strength and shock transmitting ability of the plug closure must also be increased.

In general, the plug closure can be described in terms of the characteristics of its two sections, the first section being in contact with the sample at any end surface toward which the shock wave travels, e.g., one end of a cylindrical sample, and the second section being in contact with the first section at a surface opposite that which contacts the sample. The function of the first section is to transmit the shack pressure with no substantial wave reflection and, upon transmittal, to retain the sample in the container against the pressure associated with bringing the sample to rest. The function of the second section is to carry olT the momentum associated with the shock pressure wave with no substantial wave reflection so as to prevent disruption of the sample by any net tension.

To transmit the shock pressure with no substantial wave reflection, the first section has a shock impedance at the shock pressure employed of about that of the sample, at least in the portion of the first section adjacent to the sample. A small gradual change or a change through small increments, can be tolerated as long as the impedance is about that of the sample at the interface with the sample.

To provide the requisite sample retention after the transmittal ofthe shock wave, the first section must be solid and of sufficient strength after the shock compression to retain the sample. Thus, the first section should be of material the shock induced temperature of which will be below its melting point. The first section generally will be of sufiicient strength to retain the sample if it has at least a moderate shear strength, e.g., at least about 1,000 p.s.i., after the shock compression. Before shock compression, the shear strength can be lower, e.g., when the first section initially comprises powders, granules, etc. Preferably, the shear strength will be less than about 50,000 p.s.i. High-melting solids, e.g., metals such as steel, refractory metal oxides, silicates, aluminates, carbides, silicides, nitrides, or mixtures thereof can be used in the first section. Of these, steel is preferred when the sample has a high shock impedance, e.g., on the order of about 3x10 dyne-sec./cm. at zero pressure. While the dimensions of the first section are dependent on the particular application, it has been found that, for any given material and shock conditions, varying the length of the section is a convenient way of arriving at the requisite strength, i.e., increasing the length increases the strength. Generally, a length of at least about twice the longest dimension of the cross-section of the first section normal to the direction of travel of the shock wave is preferred.

The porosity of the first section can be controlled to attain the desired impedance. In line with the permissible gradual changes in the impedance, small gradual changes or changes through small increments, in the porosity, can be tolerated as long as the requisite impedance match exists at the interface with the sample. In fact, a gradually discreasing porosity has the advantage of decreasing the post-shock temperature of the less porous part of the section, thus increasing its resistance to subsequent deformation, and is, accordingly, preferred when the shock-induced temperature of the material at the impedance-matching porosity is close to the melting point, and the strength is therefore low.

When employing shock techniques, e.g., the explosively driven circular metal cylinder technique described above, it is desirable to prevent disturbances arising from abrupt changes in the velocity of the applied pressure pulse, e.g., the driven cylinder impact velocity, in the closure region. Thus, it is desirable that the explosive surround the driver cylinder, or initial sample container where no driver is employed, throughout the length of the closure. This also is beneficial in that the contracted driven cylinder, or initial sample container, or both provide support and containment for the plug.

If the closure is subjected to the applied pressure pulse as described above, when the density and shock impedance of the first section are about equal to those of the material being shocked, the shock velocity of the wave transmitted into the plug will be equal to the shock velocity in the sample. The shock configuration in the first section, sustained by the constant axial velocity of the applied pressure pulse, e.g., the driven cylinder impact, will be substantially unchanged from that in the sample. However, if the density of the first section material is substantially greater than the density of the sample, with its impedance matching that of the sample, the transmitted shock initially will have a shock velocity substantially less than that in the sample. The constant axial velocity of the applied pressure pulse, e.g., the cylinder impact, will cause the shock configuration to distort, and will cause a shock wave with shock velocity equal to the axial velocity of the applied pressure pulse, e.g., the driven collision, to be built up at least across the central portion of the first section. Since the first section -must be of sufi'icient strength to retain the sample, the first section material must be such that this shock wave of increased pressure does not cause the first section material to be melted or heated to a temperature so near the melting point that its strength becomes to low to contain the sample. Particle velocity gradients as sociated with such a distorted shock configuration may also increase the strength requirements of the first section. Thus, the preferred situation is to have the first section of the closure approximately match the density of the sample, insofar as is consistent with a good match in impedance. If a close match of density cannot be obtained it is preferable to have the density of the first section lower than that of the sample, since this will give a transition to a lower pressure shock Wave whose velocity approaches the axial velocity of the applied pressure pulse, e.g., the cylinder impact.

To transmit the shock wave with no substantial wave reflection, th second section, at least in the portion adjacent to the first section, has a shock impedance at the shock pressure employed of about that of the portion of the first section at the interface with the second section. As with the first section, the impedance in the second section can change gradually or in small increments as long as there is the above impedance match at the interface with the first section. The second section carries off the momentum of the shock wave by spalling, i.e., separation, of enough material to contain the pressure wave, thereby preventing back transmittal of tension. One method is to cause spalling by the formation of at least a layer of melt in the second section. Melting achieves substantially complete spalling because the back transmittal of significant tension is not supported by liquids. Thus, bodies which melt below the shock induced temperature are suitable for the second section, e.g., low melting metals. In most cases, melting will be accompanied by discharging of at least a portion of the second section out of the container. This discharge provides even greater spalling.

Alternatively, the separation or spalling can be achieved by fracturing of at least part of the material in the second section, with accompanying discharging of it from the container. The second section should have a low increasing the porosity from the interface with the first section to the opposite side causes the shock pressure wave to set up a gradient of particle velocity in the second section, thereby causing at least the end portion of it to fracture and carry off the momentum. When fracture is to be employed by gradually increasing the porosity in the second section, the second section can comprise the same material as that used in the first section. This is desirable in that the requisite impedance match at the interface between the first and second sections is thereby easily obtained. Both melting and fracture can be employed in the second section to ensure carrying off of the momentum.

As noted above, when an explosively propelled cylinder is employed, the explosive and propelled cylinder are continued through the length of the closure in order to provide containment of the plug section. Similarly, when a sample container is employed initially, it and the surrounding explosive should continue through the length of the closure. The first section will tend to be held by friction with the contracted propelled cylinder. More positive retention of the plug will be achieved if the propelled cylinder, the initial sample container, or both contract to a smaller radius below the first section. This will occur in the region where spalling or separation of material in the second section occurs as a result of reflection of the shock pressure wave from the end of the closure. However, a more gradual taper of the final propelled cylinder, the initial sample container, or both will allow the first section to withstand a higher residual pressure. Before loss of the first section could occur, it 'would have to be extruded through the tapered region. Furthermore, the taper is effective in preventing leakage of any molten sample through any defects around the first section and, therefore, is especially desirable when a sample is being shocked that will tend to become molten. Such a gradual taper can be obtained by providing a gradually increasing porosity in the second section of the plug. The associated gradually decreasing impedance will not cause suflicient reflection to damage the first section.

The particle velocity resulting from passage of the shock wave will increase with increasing porosity, causing fracture and discharge of at least part of this section. Usually the increasing porosity will cause greater shock heating of the material, possibly above the melting point, thus reducing or eliminating the ability of this section to transmit tension back to the plug section by the above melting method as well as the fracture method of spalling.

While the dimensions of the second section are also dependent on the particular application, it is generally preferred that the second section have a length at least about twice the longest dimension of its cross-section normal to the direction of shock travel, e.g., twice the diameter of a circular cylindrical body having its axis parallel to the direction of shock travel.

FIGS. 1 to 3 show an illustrative shock pressure assembly, employing the novel plug closure. The sample to be shocked 1 is shown packed into circular metal cylinder 2, the top end of which is closed by plug 3, and the bottom end by the novel plug closure of this invention, 4 representing the first section and 5 representing the second section. Both 4 and 5 are circular cylinders. In this embodiment both sections comprise steel powder. Metal cylinder 2 fits coaxially inside circular metal cylinder 6, with a substantially uniform spacing 7 between the facing surfaces of cylinders 2 and 6. The coaxial spacing is maintained by metal discs 8 and 9 which abut the inner surface of metal cylinder 6 at the top and bottom ends thereof, respectively. Surrounding outer metal cylinder 6 is a thin-walled cylindrical container 11, e.g., a metal cylinder. The explosive extends across the top of the cylindrical assembly, and is initiated axially, e.g., by

'6 means of a primer 12, initiated in turn by detonating cord 13. Metal disc 14 extendsover the top end of the cylindrical assembly and is welded by cylinder 6. The entire assembly rests on metal plate 15. A circumferential notch 16 extends around the periphery of cylinder 6 in the lower region of the second section 5 of the novel plug closure. This notch assists in carrying off of the axial momentum in cylinder 6 associated with the collision with cylinder'2.

When a notch 16 is employed in the container or driver wall, it should be located around the lower portion of the second section to permit the above tapered section of the container wall to be of adequate length to have the requisite restraining strength.

The shocked sample usually will be in the form of a right circular cylinder and in these cases the plug closure will also have that form. In cases where an axial solid mandrel is passed through the sample, e.g., a supporting steel mandrel, it can also be passed axially into or through the first section of the plug closure for increased support.

The following examples serve to illustrate specific embodiments of the plug closure of this invention. However, they will be understood to be illustrative only and not as limiting the invention in any manner.

EXAMPLE 1 Referring to the drawing, cylinder 2 is made of Type 1015 steel, is 53.5 inches long, and has a 1.5-inch outer diameter and a 0.095-incl1 wall thickness. Cylinder 6 is made of Type 1015 steel, is 54 inches long, and has an 3.5- inch outer diameter and a 0.37-inch wall thickness. Spacing 7 is 0.625 inch.

At one end of cylinder 2 (the top end) is a 3.00-inchlong plug 3 consisting in this case of 6 superimposed 1.31- inch-diarneter, 0.5-inch-thick pellets of IOO-mesh lowcarbon steel powder pressed to the following densities, respectively, starting from the'outermost pellet: 82, 79.5, 77, 75, 74, and 74% (of the theoretical). Abutting this plug is the sample to be shocked 1. This consists of a mixture of 8% by weight of natural graphite having an average particle size of 2.5 and 92% by weight of copper shot of a size such as to pass a ISO-mesh, and be held on a ZOO-mesh, screen (74405 The graphite/copper mixture is pressed into pellets from a dried water slurry containing 2% guar gum. The density ofthe graphite in the sample is computed to be 50% from a volume obtained by assuming the larger-size copper to have been compacted to density, and subtracting the calculated volume of the copper from the known volume of the total mixture. The pellets are placed in cylinder 2 so as to form a solid cylinder having 1.3l-inch diameter and 30.5- inch length, and weighing 3890 grams. After having been loaded into the cylinder, the sample is heated at about 250 C. for several hours. This decomposes the gum.

Abutting the bottom of the sample is the novel plug closure of this invention consisting of two sections, 4 and 5, having a total length of 20 inches. The plug closure consists of a number of 0.5-inch-thick superimposed stee powder pellets (same powder as in plug 3). In section 4, the retaining section, which is 13 inches long, there are six pellets having densities (starting from the sample end) of 74, 74, 75, 76, 77, and 78% of the theoretical, respectively, followed by 20 pellets having a density of 79.5% of the theoretical. In section 5, the spall section, which is 7.0 inches long, there are 14 pellets decreasing in density (starting from plug 4) from 79.5 to 50% as follows: 79.5, 77, 74, 71, 68, 65, 62, 59, 56, 53, 50, 50, 50, and 50%. In section 4, the density and shock impedance of the steel are matched to the density and shock impedance of the sample adjacent thereto (approximately 75% steel density), thereby preventing reflection of the shock waves. The first section remains solid during the process and retains the sample. The gradually decreasing density in section 5 carries off the momentum associated with the shock wave without reflecting a tension wave. The momentum is carried off by fracturing and accompanying discharging of material out of the cylinder 2. The decreasing density also causes cylinder 2 to be driven infurther as the density decreases, thereby restricting the forward motion of the plugI The circumferential notch 16 is /8 inch deep and wide, and is located 1.5 inches from the bottom end of cylinder 6. This also carries E axial momentum.

Explosive is a 60-inch-high cylinder 16 inches in di ameter of a uniform mixture of grained 80/20 amatol (80% ammonium nitrate/% trinitrotoluene) and 32% sodium chloride (table salt) based on the total weight of the composition (total weight: 509 pounds). The explosive is contained in a cardboard cylinder 11 coaxial with the cylindrical assembly. The explosive is initiated axially at the top end of the assembly, five inches above a Z-inch-thick steel cover welded to cylinder 6, by means of an I-IDP-l primer (see Du Pont Blasters Handbook, 15th ed., 1966, p. 66). The detonation velocity of the explosive is 4710 meters per second.

Detonation of the detonating cord 13 initiates the primer and, in turn, the cylindrical explosive layer. Dur- 1 ing detonation of the explosive, the bottom portion of spall section 5 fractures and discharges from the container and section 4 moves downward about 3 inches. The bottom 1.5-inch section of the propelled (outer) cylinder shears otf at the notch and the two cylinders become bonded to each other. The entire sample is recovered intact by cutting open the composite cylinder to expose the sample section. Examination of the sample reveals that a substantial portion of the copper has melted and resolidified during shock treatment.

EXAMPLE 2 'Example 1 is repeated, only the graphite/copper mixture is pressed to get a graphite density in the sample of 60%. When cylinder 2 is loaded, the weight is 4108 grams. In section 4, all 26 steel powder pellets have a density of 79.5% of the theoretical.

After detonation of the explosive, the bottom portion of section 5 fractures and discharges out of the container and section 4 moves downward about 2.5 inches. Otherwise, the results are as in Example 1.

EXAMPLE 3 In Example 2, when the steel powder pellets of section 5 are replaced with the sample pellets, i.e., the graphite/ copper pellets having a graphite density of 60% the copper is melted by the shock wave and discharges out of cylinder 2. Section 4 moves downward about 2.5 inches. Otherwise the results are as in Example 1.

What is claimed is:

1. In a cylindrical container having an end closure and containing a sample being subjected to a shock wave moving in the direction of the longitudinal axis of the cylinder the improvement comprising an end plug closure capable of retaining the sample while carrying off the momenturn associated with the shock wave witha minimum of reflected waves, said closure comprising (a) a first section, in contact with said sample, having about the same shock impedance as the sample at the interface therewith at the shock pressure employed, and comprising material which, after the passage therethrough of said shock wave, is solid and of sufiicient strength to retain the sample in said container; and

(b) a second section, in contact with said first section at the surface opposite that which contacts said sample, having a shock impedance at the interface with said first section of about that of the first section at the interface with the second section at said shock pressure, and comprising material that will carry off, by spalling, momentum associated with said shock wave.

2. The container of claim 1 wherein said first section of said closure has about the same bulk density as the sample.

3. The container of claim 1 wherein said first section of said closure has a bulk density not substantially greater than that of the sample.

4. The containers of claim 4 wherein said second section of said closure comprises material that will carry off said momentum by melting.

5. The container of claim 3 wherein said second section of said closure comprises material that will carry off said momentum by fracturing.

6. The container of claim 5 wherein said second section of said closure has a gradually increasing porosity from the interface with said first section to the surface of said second section opposite saidfirst section.

7. The container of claim 6 wherein said second section of said closure is a right circular cylinder having a length of at least about twice its diameter.

8. The container of claim 7 wherein said first section of said closure has a shear strength of about from 1,000 to 50,000 p.s.i. after the passage therethrough of said shock wave.

9. The container of claim 8 wherein said first section of said closure is a right circular cylinder having a length of at least about twice its diameter.

10. The container of claim 9 wherein said first section of said closure has about the same bulk density as the sample.

11. The container of claim 9 wherein said first section of said closure comprises steel powder.

12. The container of claim 11 wherein said second section of said closure comprises steel powder.

References Cited UNITED STATES PATENTS 3,022,544 2/ 1962 Coursen et al.' 18-5I 3,128,732 4/ 1964 Paynter et 'al. 72explosive dig 3,178,807 4/1965 Bergmann 26484(UX) 3,220,103 11/ 1965 Simons 29421E 3,364,561 1/1968 Barrington 29-421EX 3,462,797 8/1969 Asburg 18-51(UX) HOWARD FLINT, JR., Primary Examiner U.S. Cl. X.R. 29-421; 264-84 

